Examples of CPB systems include Scanning Electron Microscope (SEM) systems, Focused Ion Beam (FIB) systems and hybrids that include both CPB types, which are commonly known as “Dual Beam” or “Cross Beam” microscope systems. A Focused Ion Beam system is commonly referred to as a FIB. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the SEM, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. These ions are, in general, positively charged. Note also that CPB systems may include multiple ion beams or multiple electron beams, perhaps in combination with each other.
These ion beams, when directed onto a sample, will eject charged particles, which include secondary electrons, secondary ions (i+ or i−) and neutral molecules and atoms from the exposed surface of the sample. By moving the beam across the sample and controlling various beam parameters such as beam current, spot size, pixel spacing, and dwell time, the FIB can be operated as an “atomic scale milling machine,” for selectively removing, or sputtering, materials wherever the beam is placed. The dose, or amount of ions striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The ejected particles can be sensed by detectors, and then by correlating this sensed data with the known beam position as the incident beam interacts with the sample, an image can be produced and displayed for the operator. The imaging capability of FIB systems, and of similar CPB systems, is advantageous for many applications where it is necessary or beneficial to analyze structures or features having nano scale sizes.
FIG. 1 is a schematic of a typical CPB system 10. This CPB system 10, also referred to as a dual beam or cross beam system, includes a vertically mounted SEM column and a focused ion beam (FIB) column mounted at an angle from vertical (although alternate geometric configurations also exist). A scanning electron microscope 41, along with power supply and control unit 45, is provided with the dual beam system 10. An electron beam 43 is emitted from a cathode 52 by applying voltage between cathode 52 and an anode 54. Electron beam 43 is focused to a fine spot by means of a condensing lens 56 and an objective lens 58. Electron beam 43 is scanned two-dimensionally on the sample by means of a deflection coil 60. Operation of condensing lens 56, objective lens 58, and deflection coil 60 is controlled by power supply and control unit 45. Electron beam 43 can be focused onto sample 22, which is on movable stage 25 within lower chamber 26. When the electrons in the electron beam strike sample 22, various types of electrons are emitted. These electrons may be detected by various detectors within the electron column or they may detected by one or more electron detectors 40 external to the column.
Dual beam system 10 also includes focused ion beam (FIB) system 11 which comprises an evacuated chamber having an upper neck portion 12 within which are located an ion source 14 and a focusing column 16 including extractor electrodes and an electrostatic optical system. The axis of focusing column 16 is tilted at an angle, such as 54 degrees from the axis of the electron column by example. The ion column 12 includes an ion source 14, an extraction electrode 15, a focusing element 17, deflection elements 20, and a focused ion beam 18. Ion beam 18 passes from ion source 14 through column 16 and between electrostatic deflection means schematically indicated at 20 toward sample 22, which comprises, for example, a semiconductor device positioned on movable stage 25 within lower chamber 26.
Stage 25 can preferably move in a horizontal plane (X and Y axes) and vertically (Z axis). Stage 25 can be tilted and rotated about the Z axis. A door or load lock 61 is opened for inserting sample 22 onto X-Y stage 25 and also for servicing an internal gas supply reservoir, if one is used. The door is interlocked so that it cannot be opened if the system is under vacuum.
An ion pump 28 is employed for evacuating neck portion 12. The chamber 26 is evacuated with turbomolecular and mechanical pumping system 30 under the control of vacuum controller 32. The vacuum system provides within chamber 26 a vacuum of between approximately 1×10−7 Torr and 5×10−4 Torr. If performing gas assisted processes such as etching or deposition, an etch retarding gas, or a deposition precursor gas is used, the chamber background pressure may rise, typically to about 1×10−5 Torr.
The high voltage power supply provides an appropriate acceleration voltage to electrodes in ion beam focusing column 16 for energizing and focusing ion beam 18. When it strikes sample 22, material is sputtered, that is physically ejected, from the sample. Alternatively, ion beam 18 can decompose a precursor gas to deposit a material on the surface of the sample.
High voltage power supply 34 is connected to liquid metal ion source 14 as well as to appropriate electrodes in ion beam focusing column 16 for forming an approximately 1 keV to 60 keV ion beam 18 and directing the same toward a sample. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20 whereby ion beam 18 may be controlled manually or automatically to trace out a corresponding pattern on the upper surface of sample 22. The liquid metal ion source 14 typically provides a metal ion beam of gallium. The source typically is capable of being focused into a sub one-tenth micrometer wide beam at sample 22 for either modifying the sample 22 by ion milling, enhanced etch, material deposition, or for the purpose of imaging the sample 22. Note that newer source technologies such as plasma, gas field ion sources and/or atomic level ion sources will produce other ionic species besides gallium.
A charged particle detector 240 used for detecting secondary ion or electron emission is connected to a video circuit 42 that supplies drive signals to video monitor 44 and receiving deflection signals from controller 19. The location of charged particle detector 40 within lower chamber 26 can vary in different configurations. For example, a charged particle detector 40 can be coaxial with the ion beam and include a hole for allowing the ion beam to pass. In other configurations, secondary particles can be collected through a final lens and then diverted off axis for collection.
A micromanipulator 47 can precisely move objects within the vacuum chamber. Micromanipulator 47 may include precision electric motors 48 positioned outside the vacuum chamber to provide X, Y, Z, and theta control of a portion 49 positioned within the vacuum chamber. The micromanipulator 47 can be fitted with different end effectors for manipulating small objects.
A gas delivery system 46 extends into lower chamber 26 for introducing and directing a gaseous vapor toward sample 22. For example, xenon difluoride can be delivered to enhance etching, or a metal organic compound can be delivered to deposit a metal.
A system controller 19 controls the operations of the various parts of dual beam system 10. Through system controller 19, an operator can control ion beam 18 or electron beam 43 to be scanned in a desired manner through commands entered into a conventional user interface (not shown).
In recent years, two and three dimensional imaging of large areas and volumes in a charged particle beam system such as SEM, FIB, or SEM/FIB combination microscope has attracted significant interest. Commercial systems such as the Carl Zeiss ATLAS two dimensional imaging system along with three dimensional imaging systems such as the FEI Company “Slice and View” along with methods described in U.S. Pat. No. 7,312,448 B2 have been available commercially. These techniques are generally performed on “bulk” samples, where the charge particle beam penetrates but does not transmit through the sample. It should be noted that this is quite different from the technique of electron tomography, which relies on the charged particle beam passing through the sample in transmission. While electron tomography is a well established technique in transmission electron microscopy, and can yield three dimensional datasets, these datasets are limited in scale due to the necessity of passing the electron beam completely through the sample and detecting it on the other side.
The aforementioned “ATLAS” two-dimensional and “Slice and View” style three dimensional techniques are sophisticated in their own right, however they both approach the problem of acquiring large datasets in a similar “step and repeat” fashion. In both cases a two-dimensional area is imaged either as a single image or as a collection of image “tiles” that may be “stitched” together to form a larger mosaic. Two-dimensional techniques tend to perform this step and repeat imaging over much larger areas than three-dimensional techniques, however three-dimensional techniques also remove a thin “slice” of material, then repeat the imaging process so as to build up a three dimensional dataset. This slice of material may be removed in several ways known in the art, including the use of a focused ion beam (typically at glancing angle, but occasionally closer to normal incidence), a broader ion beam which is often combined with some sort of mechanical beam stop to create a sharp edge, or an in-microscope ultramicrotome whose knife cuts away each slice.
CPB systems, such as FIBs or SEMs, have been used prevalently in the past for imaging small regions of a sample at high resolution. In the field of semiconductor circuits for example, typical structures being imaged include transistor devices and other small structures having dimensions from a few nanometers up to a few microns. In recent years, bio-medical applications are emerging in which higher resolution images for a large area of a sample are desired using the aforementioned 2D and 3D imaging techniques, and combining FIB and SEM. For example, imaging of a tissue sample having an area of 100×100 microns may be required in order to facilitate visual identification of a particular structure of interest which may be present. Accordingly, a high resolution image of the entire area is required, otherwise it may not be possible to visually identify the structure of interest. Furthermore, the particular structure of interest may lie within a plane different from the exposed area being imaged. In this example, if the imaged area of the sample is defined by an x-y plane, then the tissue sample has a depth component, z. Therefore sections of the tissue sample are taken at predetermined depths and the newly exposed area is imaged.
The problem with currently known techniques is the large amount of time required to image large volume samples at high resolution. The increasing demand for 3D high resolution images of 100 μm×100 μm×100 μm volume samples is problematic. Typically sections are prepared ˜15 μm Sections@˜3 nm pixels with ˜9 nm depth per slice using a FIB; more typically three times this depth per slice is all that can be achieved using other sectioning methods which can be used. Typical dwell times for the electron beam are on the order of 1 μs per pixel in order to obtain sufficient signal to noise. At 3 nm voxels with a dwell point time of 1 μs, 20 minutes of imaging time alone per section are required, and about 110 hours per μm of depth sectioned, which must be multiplied by 100 to section through 100 μm of depth, and this is imaging time alone, i.e. it is assumed sectioning occurs concurrently or near instantly. Therefore a total of about 1.5 years of time is required to image a 1,000,000 μm3 cube, assuming that the CPB system is capable of operating for this continuous period of time without malfunction or interruption, or the sample undergoing sectioning can be reacquired and realigned in an acceptable manner. Another issue related to imaging large areas is the fact that the sample is vulnerable to “drifting” during the imaging process, in which the sample moves due to mechanical variations in the stage supporting the sample, and/or thermal effects on the environment of the microscope.
It is, therefore, desirable to provide a method and system for reducing the amount of time required for CPB imaging while maintaining accuracy.